But Moy Park boss Janet McCollum declared that "Europe was their market". The firm has operations in the Republic, and in England, as well as in Netherlands and France.

She said: "Within the EU, UK companies have open access to 500 million consumers and, in the event of a Brexit, no-one has been able to guarantee our continued access to those markets.

"There are also wider issues regarding access, cost volatility and the complications in dealing with animal welfare standards.

"The EU has imperfections but it is a massive market which offers export security and the UK needs to remain within it."

Moy Park has benefited hugely from freedom of movement across the EU and is estimated to employ over 1,000 EU migrants from countries such as Lithuania, Poland and Portugal in its Northern Ireland facilities.

In France, it employs 800 people in three locations, and 6,500 in Northern Ireland. There are around 5,400 staff in England, 50 in the Republic and 100 in Holland.

Randox, which employs around 900 people, won the Ruban D'Honneur for exports in the European Business Awards 2014 to 2015.

Mr FitzGerald said: "It's a complex issue. Randox is a UK-based company and we export around the world.

"However, this is a decision for the British electorate. Whatever the outcome, at Randox we will seek to maximise trade, exports, and job creation."

And Jim Burke, the Northern Ireland head of Montupet, has said misleading information on both sides of the Brexit debate risks creating "dangerous territory".

The French-owned firm manufactures cylinder heads for the car industry and employs 600 people in Dunmurry.

Mr Burke said the firm benefited from "substantial EU funding" and said it supported EU membership - provided the union was "run properly".

He said the issues around membership were so complex that the public was likely to be swayed by emotive arguments. "This is very dangerous territory and is tantamount to playing Russian roulette without knowing how many bullets are in the chamber."

And William Barnett, the chief executive of W&R Barnett, said it was in favour of staying in the EU - adding that the risks of leaving were particularly severe for Northern Ireland.

"As a company that operates in a number of European markets we have benefited from the single market," he said.

"The ability to trade, operate and invest safely in neighbouring EU countries under a common set of rules is a valuable asset and not something that should be taken for granted.

"Although there are frustrations within our business with the EU, it is also our experience that some of the typical areas of business frustration with 'EU red tape' would be better directed at local and national government," he added.